A major cause of death and disability is the failure of the cardiac left ventricle to adequately perfuse the body with blood. While numerous disease conditions cause left ventricular failure, coronary artery disease is perhaps the most common. Regardless of the cause, ineffectual ejection from the left ventricle further weakens the heart since the heart receives blood supply through coronary arteries during the resting phase. As the heart muscle loses the ability to pump blood, both systolic and diastolic blood pressures are reduced with a reduced diastolic pressure. The pressure maintained in the arteries during the time of heart muscle relaxation is reduced thereby limiting blood flow through the coronary arteries. With reduced blood flow through the heart itself, the heart muscle is further deprived of essential nutrients.
The prior art includes numerous cardiac assist devices that work in concert with a failing heart in order to increase cardiac ejection volume and pressure. One form of cardiac assist device that has met with some success is the left ventricular assist device (LVAD) that is a secondary pump located in operative communication with the aorta in order to improve blood flow characteristics from an adjacent failing heart. A left ventricular assist device is known in the art to take the form of an intra-aortic balloon pump, a patch pump sutured to the aortic wall, and a cuff constrictive about the aorta. A common feature of all these variants of LVADs is a pulsatile pumping that is phase synchronized to coincide with the point of inflection associated with the end of the cardiac systolic phase and the beginning of diastole. This point of inflection is commonly referred to as the dicrotic notch. The timing of LVAD pumping in a counter-pulsatile fashion that commences at the dicrotic notch is critical in optimizing auxiliary pumping effectiveness. Physiologically, the dicrotic notch correlates with closure of the aortic valve.
The electrocardiogram (EKG) cannot accurately predict the timing of the dicrotic notch since the time delay in the EKG reading relative to aortic closure is unknown. Prior art attempts to measure the time delay in EKG for determining the dicrotic notch have included blood pressure monitoring. Representative blood pressure monitoring schemes are provided in U.S. Pat. Nos. 4,077,394 and 6,623,420. Unfortunately, implanted pressure sensors tend to foul over time while noninvasive pressure sensors tend to shift position and detract from patient quality of life.
As an alternative approach an implanted microphone has been contemplated in sensing aortic valve opening in the context of LVAD based on direct intrinsic compression of the ventricle, as exemplified in U.S. Pat. No. 6,251,061 B1. However, ferrofluid pumping to compress the heart remains an untested technology that is surgically intensive to implement.
Thus, there exists a need for a system to control counter-pulsation in an LVAD with precise timing control relative to the closure of the aortic valve and independent of a pressure sensor.